Opening Thoughts on Resistance

Understanding the intricacies of resistor layout will allow you to catch mistakes, improve CAD tools, and read unfamiliar layouts. It allows you shortcuts, like building a diode with a resistor in series without having a separate diode and separate resistor. You can modify circuit elements in your layout to get new creations that do some unique tricks for you.

You will need to know how resistors are constructed if you want to get into such advanced areas as rules file writing or extractions files. For example, if you are trying to write a rules file to extract a specific resistor type from a layout, you have to know what uniquely defines the various chunks. For example, if I find the P, the P+, and the active touching my poly layer, then I have an FET.

I recognize quickly it is not my certain resistor and I can throw all that away. If I have my poly layer without an active touching it, then I know I have a resistor.

Many people just use the automatically generated components. However, you might need to develop resistor layout for a brand new process. For example, someone might say, "Ok, go lay out a bunch of diffusion resistors." Oh, crikey, there are no rules! No one has done it before! You have to understand what the layers are, how they're used, and where they fit in.